Field of the Invention
The present invention relates to a measurement method and also to an electron microscope.
Description of Related Art
A scanning transmission electron microscope (STEM) is an electron microscope for obtaining scanning transmission electron microscope (STEM) images by scanning a focused electron beam over a sample, detecting a signal originating either from electrons transmitted through the sample or from scattering electrons, and mapping the intensities of the signal in synchronism with the scanning. In recent years, scanning transmission electron microscopes have attracted attention as electron microscopes capable of providing quite high spatial resolutions at the atomic level.
A segmented detector whose detection surface is divided into plural detector segments is known as an electron detector equipped in such a scanning transmission electron microscope. The segmented detector has independent detection systems for the detector segments, respectively. Each detection system detects only electrons striking a respective one of the detector segments on the detection surface. A scanning transmission electron microscope performs imaging while bringing the detection surface into coincidence with the diffraction plane. That is, this is equivalent to detecting electrons transmitted and scattering within a certain solid-angle region from a sample. Consequently, this presents the advantage that the use of a segmented detector permits one to simultaneously measure the solid angle dependence of scattering of electrons caused by the sample and to obtain a quantitative evaluation (see, for example, JP-A-2011-243516).
FIG. 15 illustrates the operation of a scanning transmission electron microscope 101 equipped with a conventional segmented detector. Note that only main portions of the microscope 101 are shown in FIG. 15.
In the scanning transmission electron microscope 101, as shown in FIG. 15, an electron beam EB is focused onto the surface of a sample S by an illumination lens system 102. The camera length is adjusted by an imaging lens system 104 for the electron beam EB transmitted through the sample S. Then, the beam is detected by the segmented detector, 106. A CCD camera 108 is positioned behind the segmented detector 106.
The differential phase contrast (DPC) technique is known as a technique of visualizing an electromagnetic field produced in a sample using a scanning transmission electron microscope equipped with such a segmented detector. In this technique, an amount by which an electron beam is deflected when it passes through a sample is measured, and the electromagnetic field in the sample causing the deflection is computed.
When a measurement is made using the DPC technique, it is necessary to align the directions of the detector segments of the segmented detector to an STEM image. If the directions of the detector segments relative to the STEM image are not known, then it is impossible to identify the direction of an electromagnetic field that acts on the electron beam transmitted through the sample to thereby deflect the beam.
FIG. 16A is a diagram illustrating one example of the relationship between the crystallographic orientation of the sample S and the orientations of detector segments D1, D2, D3, and D4 of the segmented detector 106. FIG. 16B schematically shows an image I(D2-D4) produced by taking the difference between STEM images obtained from the detector segments D2 and D4, respectively, as well as an image I(D1-D3) produced by taking the difference between STEM images obtained from the detector segments D1 and D3, respectively.
As shown in FIG. 16A, the detector segments D1-D4 are so arranged that the detector segments D2 and D4 are aligned in the [110] direction and that the detector segments D1 and D3 are aligned in the [110] direction. Under this condition, STEM images are taken from the detector segments D1, D2, D3, and D4, respectively. Then, the image I(D2-D4) and the image I(D1-D3) shown in FIG. 16B are generated. The X direction of the captured STEM images lies in the [110] direction of the sample S, while the Y direction of the STEM images lies in the [110] direction of the sample S.
Information about the deflection in the [110] direction produced when the electron beam passes through the sample can be obtained from the image I(D2-D4) shown in FIG. 16B. Information about the deflection in the [110] direction can be derived from the image I(D1-D3). The distribution of the electromagnetic field, for example, in the sample can be known from the relationship between the crystallographic orientation and the deflection.
A conventional method of measuring the directions of the detector segments D1, D2, D3, and D4 of the segmented detector 106 relative to the STEM images is now described by referring to FIGS. 17-22. In order to know the directions of the detector segments D1, D2, D3, and D4 of the segmented detector 106 relative to the STEM images, the imaging lens system 104 is first adjusted. A setup is made such that the surface of the sample is conjugate with the detection surface 105 as shown in FIG. 17. Under this condition, if scanning is done, an image I1 of the shape of the detection surface 105 can be obtained as shown in FIG. 18. If the segmented detector 106 is retracted and an image I2 (see FIG. 19) of a probe that is scanned by the CCD camera 108 is obtained, the active detector segment being scanned can be confirmed. By combining these images, the azimuthal relationship of the detector segments D1, D2, D3, and D4 of the detection surface 105 to the CCD camera 108 can be known as shown in FIG. 20.
Then, the setup for obtaining STEM images as shown in FIG. 15 is resumed. Under this condition, an image I4 of a shadow of an aperture (not shown) of the illumination system is observed as shown in FIG. 21A. If the image is defocused and the detection surface 105 is moved off the diffraction plane, the shadow of the aperture moves along with the scanning. For example, if a scan is made only in the X direction and an image capture is made with a long exposure time, an image I5 indicating a trajectory of the shadow of the aperture is obtained as shown in FIG. 21B. The directions of the detector segments D1, D2, D3, and D4 of the segmented detector 106 relative to the STEM image are known as shown in FIG. 22 from the direction of motion a of the shadow of the aperture and from the directions of the detector segments D1-D4 of the detection surface 105 appearing in an image I3 shown in FIG. 20. The direction a shown in FIG. 22 is the X direction of the STEM image.
In this way, in the conventional method of measuring the directions of the detector segments relative to an STEM image, two operations have been performed. In the first operation, the sense of the CCD camera relative to the segmented detector is measured. In the second operation, the sense of the CCD camera relative to the direction of scanning is measured. Therefore, in the conventional measurement method, it is time-consuming simply to obtain the images I1-I5 needed for a measurement. The directions of the detector segments of the segmented detector relative to an STEM image vary simply when the scanning direction is changed. Therefore, if the above-described measurement is performed whenever the scanning direction is varied, a heavy burden is placed on the user. Furthermore, the conventional measurement method needs a CCD camera.